This invention relates to an improved method of recovery of oil from a subterranean oil-bearing formation utilizing in-situ combustion techniques. More particularly, this invention relates to the sequential establishment of air permeability in individual well pattern units that comprise a plurality of wells traversing the formation prior to the initiation of in-situ combustion, thereby increasing the sweep efficiency and resulting in increased oil recovery.
Oil is initially recovered from most subterranean oil-bearing formations by utilizing the natural energy contained therein, such as solution gas, gas pressure or natural water drive, whereby the formation oil is displaced through the formation to producing wells from which it is recovered. In most primary recovery operations the primary energy is depleted before the in-place oil has been recovered. Considerable oil, up to 75%, may remain unrecovered in the formation. Thus, it is the usual practice to undertake a secondary oil recovery method whereby additional oil may be recovered from the formation by providing additional energy to the formation, in the form of a drive mechanism.
One of the secondary recovery techniques that is employed is that of in-situ combustion, which involves combustion of a portion of the oil in the formation. In the conventional in-situ combustion method, applied to a formation traversed by at least one injection well and a spaced production well or wells, an oxygen-containing gas, such as air, is injected into the formation via the injection well traversing the formation and combustion of a portion of the oil adjacent the injection wellbore is initiated by one of a number of well-known methods such as a down-hole burner, a gas-fired or electric heater, or the use of chemicals. A high-temperature combustion front is formed, which is maintained and displaced through the formation toward the spaced production well or wells by the continued injection of the oxygen-containing gas. As the combustion front progresses through the formation the in-place oil is displaced ahead of the front toward the production well or wells from which the displaced oil is recovered.
In the application of in-situ combustion to large formations, a plurality of well pattern units may be used in which each unit comprises a central well and spaced offset wells. For example, the pattern units may be 5-spots in which the central well is an injection well and the four corner or offset wells are producing wells. The in-situ combustion operation may be applied to each 5-spot pattern simultaneously or individual 5-spot patterns may be produced in a phased sequence, dependent upon the capabilities and limitation of the air compressor units. In another scheme of operation, a line drive may be utilized wherein one row of wells serves as the injection wells and the two rows of wells on either side of the line of injection wells serve as producing wells. With this pattern, generally all of the injection wells are ignited at one time so as to develop a line drive combustion front.
In the portions of the formation swept by the combustion front, recovery, ideally, approaches 100% except for that fraction of the oil consumed as fuel to sustain the combustion. This fraction is generally in the range of 10% to 20% of the oil in place. While, ideally, excellent recovery is possible, in practice considerably less recovery is realized. Among the reasons for the lower recovery is that of poor sweep efficiency. Sweep efficiency is defined as the ratio of the area of the formation or pattern within the pathways of travel of the displacing fluid (i.e., air) to the total area of the formation or pattern. Practical operations from the standpoint of economics require a maximum sweep efficiency.
Poor sweep efficiencies may be attributable to anomalies in the formation characteristics such as permeability variations and porosity variations. For example, the combustion front may be channeled through the formation along paths of high permeability, thereby by-passing considerable oil in the formation instead of sweeping the oil as a bank from the injection well to the producing well.
Poor sweep efficiencies may also be caused by the movement of the combustion front itself. As the combustion front progresses outwardly into the formation, it changes shape from that of an ideally radial movement to one in which distortion occurs because of the pressure sinks around the producing wells. These sinks cause a portion of the front to accelerate and "cusp" toward the producing wells resulting in undesired early breakthrough of the combustion front at the producing wells. Once breakthrough occurs, the size of the area of the formation swept by the front is essentially fixed since subsequently injected air will travel almost entirely through the low resistance paths already available rather than try to force new channels through the formation. Consequently, considerable portions of the formation may be by-passed and thus the in-place oil is not recovered.
Prior art has suggested solutions to the problem of poor sweep efficiency by attempting to control the undesirable, irregular advancement of the front by, among other things, creating more favorable mobility ratios during the operation. It is known that sweep efficiency is dependent upon mobility ratio. The mobility ratio is defined as the ratio of the mobility of the displacing fluid divided by the mobility of the displaced fluid. Mobility, in turn, is dependent upon the viscosity of the fluid and the relative permeability of the formation to the fluids. With a more favorable mobility ratio, i.e., 1 or less, better sweep efficiency may be attained. One method of attaining a more favorable mobility ratio is to inject water alternately with the injected air.
Improved sweep efficiency may also be realized by inhibiting cusp formation. It is known that the geometry of well patterns or rate distribution may be used to control distortion of the front. Two principal means of accomplishing cusp retardation are (a) "pinning the cusp" by locating inner producing wells between the injection source and the outer producing wells and keeping these inner or control wells on production after breakthrough, and (b) "spreading the cusp" by pulling the front toward side wells until breakthrough thereat before allowing the front to proceed in the direction of the corner producing wells of a pattern unit.
In addition, there are many teachings for improving sweep efficiency involving geometry patterns, well positions and injection sequences. For example, U.S. Pat. No. 3,472,318 teaches a method of production in whicn in-situ combustion is initiated in an inverted 5-spot pattern unit. After breakthrough occurs at one of the producing wells, the well is converted to an injection well to receive the produced water, and the remaining wells are put on a stand-by basis, and production is commenced at a well adjacent the recently converted injection well. In U.S. Pat. No. 3,999,606 a method for improving in-situ recovery is taught wherein the pressure in the locus ahead of the front is increased by throttling the producing wells to selectively retard the front movement.
While these suggested methods to improve sweep efficiency have been applied and address themselves to the operation of the in-situ combustion itself, we have now found that improved sweep efficiency can be realized if, prior to the initiation of in-situ combustion, a conditioning of the formation is undertaken whereby the air permeability of the formation is improved so that the formation thereafter is more uniformly receptive to air for the ensuing in-situ combustion operation.